The structure
of a compound has a big effect on its properties. But how do we know what that
structure is?

The most useful methods of determining molecular structure involve the
interaction of electromagnetic radiation, or light, with matter. Visible light,
ultraviolet and infrared radiation, and even microwaves and radio waves interact
with matter. They can each tell us different kinds of information about the
materials they interact with.

How does light interact
with matter?

Light has wave properties, much like the waves you could see at the ocean
shore. In physics, waves can be described in a number of different ways.
Waves have amplitude: there are waves that rise very tall, and others that are
low. Waves also have wavelength: they may have long wavelengths,
with big distances from the peak of one wave to the peak of the one coming
behind it. They may have short wavelengths, with one following very
closely behind another. Wavelength gives rise to a complementary property,
which is frequency. WHen waves are close together, you can see or hear
them crashing to the shore very frequently. When they are farther apart,
they seem to crash to the shore with a much lower frequency.

The different colors of light that we see have different wavelengths;
blue light has a shorter wavelength than red light, for example. These
different wavelengths of light have different amounts of energy. This idea is
described in the Planck-Einstein relation:

E = h ν

(where E = energy, h =
Planck's constant, n = frequency)

or

E = h c / λ

(where c = speed of light,
λ = wavelength)

This equation means:

Higher frequencies of light are
more energetic than lower frequency ones (when the number ν gets bigger, the number E also gets bigger).

When
ultraviolet and visible light are absorbed, the energy from the light is
transferred to an electron. The electron is excited to a higher energy level.
Only certain energy levels are available in a material, and so the material can
only absorb certain photons. That means:

A photon with not enough energy to reach another energy level is
not absorbed.

A photon with too much energy to reach another energy level is
not absorbed, either; the
electron cannot absorb some of the energy from a photon and have a little
left over for later.

The wavelength or frequency of a
photon that is absorbed by the electron corresponds to the amount of
energy needed to reach another energy level.

The same sort
of event can happen "backwards": an electron can lose energy by falling to a
lower energy level. The lost energy can be given up by the electron as a photon
of light. The wavelength or frequency of the photon corresponds to the
difference between electron energy levels. This phenomenon, in which light is
absorbed by a material and then given off again, is called "fluorescence".

There are many kinds of electromagnetic radiation.

Many of these kinds of "light" can provide different
kinds of information about structure. For example:

X-rays can be used to construct an
exact three-dimensional map of where the atoms lie in a crystalline material
based upon how the x-rays scatter as they pass through the crystal. X-ray
crystallography is a little bit too complicated for us, however.

Radio waves interact with nuclear
particles in a way that is similar to the absorption of UV light by electrons.
However, this phenomenon only occurs in a strong magnetic field. The absorption
of radio waves by the hydrogen nuclei in water molecules in human tissues is
referred to as magnetic resonance imaging (MRI). The observation of nuclei in
small molecules by a similar technique is referred to as nuclear magnetic
resonance (NMR). NMR spectroscopy will be the subject of
another chapter.

Infrared light is absorbed by
different bonds in a molecule. Infrared spectroscopy is the subject of
another
chapter.

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's
University (with contributions from other authors as noted). It is freely
available for educational use.